Michael Pizzo (Inactive) added a comment - 17/Jan/13 2:14 AM - edited The rules are not clearly defined in the contribution, but the intent was that, if metadata:type is not specified, the type is assumed to be string. So it is optional for primitive types, but its absence implies string. ATOM was intended to be self-describing. Currently you do not need metadata in order to interpret the payload. Not defining a specific meaning to metadata:type being missing for a primitive type would be the first case where metadata was required to interpret the ATOM payload.